Before we get down to work, let's explain what you'll learn today.
In Part 3, you learned what
PARTITION BY is. It allows you to compute certain functions independently for groups of rows and still maintain their individual character.
In Part 3, we only used
PARTITION BY with familiar aggregate functions:
SUM(). None of these functions required the use of
ORDER BY: the order of rows simply doesn't matter in this case.
However, in Part 4, 5 and 6, you got to know new elements where the order does matter: ranking functions, window frames and analytical functions.
In this part, we'll teach you how to use
PARTITION BY with these new elements. You will need an
ORDER BY clause – hence the name of this part. Remember to use the proper order:
PARTITION BY comes before
ORDER BY. Otherwise, your query won't make sense.